1. Field of the Invention
The present invention relates to a laser beam printer which scans a laser beam to reproduce an image.
2. Related Background Art
FIG. 10 is a view for explaining an image reproducing operation of a conventional laser beam printer. In the figure, an image signal (VDO) 51 is inputted to a laser unit 52 which modulates the signal to form an on-off modulated laser beam 53. A motor 54 rotates a polygon mirror 55 at a constant speed. A focussing lens 56 focusses a laser beam 57 deflected by the mirror on a photosensitive drum 58. Thus, a laser beam modulated by the image signal is scanned over the photosensitive drum 58 in the direction of horizontal scanning (in the direction of main scanning A). A beam detector 59 comprising a photoelectric conversion element 60, e.g., a photodiode, outputs a horizontal synchro signal (hereinafter called BD signal) which determines a timing of writing an image. A latent image formed on the photosensitive drum 58 is visualized by an unrepresented developing unit and it is transferred on a transfer sheet 62 at an unrepresented transfer unit using a developing agent toner).
The operation of each element will be described below.
The laser unit 52 generates a laser beam 53 modulated in accordance with an inputted image signal (VDO) 51. The VDO signal 51 is generated by an unrepresented control unit in the laser beam printer. The modulated laser beam 53 is applied to the polygon mirror 55 having a plurality of mirrors and driven by the motor 54, the applied laser beam being deflected in the horizontal direction. A constant speed revolution of the polygon mirror 55 makes the deflected laser beam 57 scan over the photosensitive drum 58 at a constant speed. The laser beam 57 is focussed on the photosensitive drum 58 by the focussing lens 56. While the photosensitive drum 58 rotates at a constant speed and the laser beam 57 scans over the photosensitive drum 58 at a constant speed, a latent image of the VDO signal 51 is formed on the photosensitive drum 58. In this case, as the laser beam 57 is applied to the photoelectric conversion element 60 of the beam detector 59 fixed near the position the laser beam 57 starts scanning, a BD signal 61 is generated from the photoelectric conversion element 60. The BD signal 61 generates once at each scanning of the laser beam 57 and has a constant period. The unrepresented control unit generates the VDO signal 51 corresponding to one scanning synchronously with the BD signal 61 to thereby define the image write start position in the main scan direction on the photosensitive drum 58.
FIGS. 11(a) to 11(d) show a timing chart illustrating the relationship between the VDO signal 51 and the BD signal 61, wherein reference numerals identical to those in FIG. 10 are used to indicate the same signals. A BD abnormality measuring signal 71 of FIG. 11(c) monitors if a VD signal 61 of FIG. 11(a) is outputted at a predetermined timing. A BD abnormality informing signal 72 of FIG. 11(d) informs of an abnormality when the BD signal 61 of FIG. 11(a) is not detected within one period of the BD abnormality measuring signal 71 of FIG. 11(c). In the Figures, T represents a time from when the BD signal is generated and to when the laser beam 57 reaches the effective image area on the photosensitive drum 58, To represents a scanning period of the BD signal 61, Ta represents a time from the trailing edge of the BD signal 61 to the trailing edge of the BD abnormality measuring signal 71, Tb represents a time from the trailing edge of the BD signal 61 to the leading edge of the BD abnormality measuring signal 71.
As seen from FIG. 11(a) to 11(d), after a lapse of a certain time T after the beam detector 59 detects the BD signal 61, the VDO signal 51 (indicated by slanted oblique lines) is generated. The timing chart corresponds to the operation during three scan lines.
As above, the latent image formed on the photosensitive drum 58 is visualized by a known electrophoto process and thereafter, it is transferred and fixed on a transfer sheet 62 to obtain a hard copy.
FIG. 12 is a plan view showing a relationship between the photosensitive drum 58 and the transfer sheet 62, wherein elements identical to those in FIG. 10 are represented by using same reference numerals.
In FIG. 12, L1 represents the left end position of an image to be formed on the transfer sheet 62, L2 represents a left margin of the transfer sheet, L3 represents a positional displacement, and L4 represents a distance from the photoelectric conversion element 60 to the left end position L1 of an image. L5 represents a distance from the photoelectric conversion element 60 to the left end of a transfer sheet 62 as set correctly. The transfer sheet 62 shown by a solid line is displaced from the correct position by L3.
Conventionally, a single photoelectric conversion element 60 has been provided on a beam selector 59 which is fixed in position relative to the photosensitive drum 58. Therefore, the left end position L1 of an image to be formed on the photosensitive drum 58 is unambiguously determined from the BD signal 61 and the VDO signal 51 shown in FIGS. 11(a) and 11(b). Particularly, to ensure the left margin L2 of an image to be transferred on a transfer sheet 62, the transfer sheet should be conveyed such that its left end passes at a distance L5 spaced apart from the photoelectric conversion element 60.
However, if a transfer sheet 62 is set displaced by L3 in the main scanning direction X as that shown by a solid line in FIG. 12, the left margin obtained on the transfer sheet 62 becomes L2-L3, and hence the same displacement in printed images in the main scanning direction. Thus, there occurs a problem of missing and changing a printed image.